Substrates of dielectric ceramic material are used in the field of electronics, in particular for manufacturing a great variety of products, such as substrates adapted to constitute supports or carriers of circuits used in the technological fields of the so-called "thin layer" and "thick layer" techniques, manufacture of protection cases for microprocessors or for microprocessor interconnections, manufacture of multilayer substrates etc.
According to the most wide-spread technique alumina is used for producing such ceramic material.
In hyperfrequency applications, the tangent of the dielectric loss angle (tg .delta.) should be small, e.g. in general smaller than 5.multidot.10.sup.-4 at 10 GHz. Reactive alumina of 99.5% purity can be sintered at temperatures comprised between 1500.degree. C. and 1600.degree. C. and may have loss angle tangent values lower than 2.multidot.10.sup.-4 at frequencies up to 10 GHz. Less pure alumina, e.g. 96% or 89% alumina which essentially contain the elements Si, Mg, Ca, Na, K can be sintered at temperatures as low as 1400.degree. C., however their loss angle tangent values are higher than 10.sup.-3 at 10 GHz.
Furthermore, according to another requirement a material used as a substrate in the field of microelectronics must have as smooth a surface as possible, the surface roughness being in the order of a micron. In other words, the surface roughness of the ceramic material should be reduced as much as possible. In addition, it is desirable to carry out the sintering treatment of the material involved at as low a temperature as possible.
The present invention allows all these requirements to be met. The invention provides indeed a ceramic substrate adapted to be sintered at temperatures lower than 1400.degree. C. The dielectric loss angle tangent tg .delta. of the ceramic material, as measured at 10 GHz, is lower than 5.multidot.10.sup.-4, and said material in the form of a substrate has a reduced surface roughness.